This is also a very good resource.
Part of the problem with looking for "original" lyrics is that the more popular tunes were published with multiple sets of lyrics, depending on which Minstrel troupe was doing the publishing. I recommend the books Dan Emmett and the Rise of Negro Minstrelsy, and Dan Partner's The Birth of the Banjo. I've attached a bibliography of a number of books and sites I used for a talk I recently gave at the Midwest Civil War Civilian's conference. Hope some of this is useful.